System for converting thermal energy to mechanical energy in a vehicle

ABSTRACT

A system for converting thermal energy to mechanical energy in a vehicle: A line circuit ( 35 ), a pump ( 36 ) for recirculating a medium in the line circuit, at least one evaporator ( 31, 32, 38 ) in which the medium is caused to absorb thermal energy from a heat source ( 4, 28 ) so that the medium becomes vaporised, a turbine ( 39 ) driven by the vaporised medium in order to generate mechanical energy, followed by a condenser arrangement ( 24, 42 ) in which the medium is caused to give off thermal energy so that it condenses. The condenser arrangement includes a first condenser ( 24 ) in which the medium gives off thermal energy to coolant which circulates in a cooling circuit, and a second condenser ( 42 ) downstream of the first condenser ( 24 ) with respect to the medium&#39;s direction of flow in the line circuit ( 35 ) and in which the medium gives off thermal energy to air at the temperature of the surroundings.

BACKGROUND TO THE INVENTION AND PRIOR ART

The present invention relates to a system for converting thermal energyto mechanical energy in a vehicle according to the preamble of claim 1.

When fuel is burnt in a combustion engine, chemical energy is convertedto mechanical energy. A substantial part of the chemical energy is notconverted to mechanical energy but to thermal energy which is releasedto the surroundings in various ways. An example of this is the thermalenergy in exhaust gases which are discharged to surrounding air. Anotherexample is thermal energy which is cooled away from various kinds ofwarm media in vehicles. An example of such warm media is the coolant inthe cooling system which cools the combustion engine. The coolant ishere usually cooled in a radiator situated at the front portion of thevehicle. Many combustion engines are supercharged. The charge air isusually cooled in at least one charge air cooler before it is led intothe engine. A known practice for reducing discharges of nitrogen oxidesis to recirculate part of the exhaust gases. The recirculating exhaustgases are cooled in at least one EGR cooler before they are recirculatedto the engine.

WHR (waste heat recovery) systems are used to convert thermal energy tomechanical energy. A WHR system comprises a circuit with a pump whichcirculates a medium in a line circuit. The line circuit comprises anevaporator in which the medium is vaporised by heat from a heat source,and a turbine which is driven by the vaporised medium. When the mediumexpands through the turbine, part of the medium's thermal energy isconverted to mechanical energy which may be utilised for directoperation or be converted to electric energy. The medium condenses in acondenser. It is important that the medium be cooled to a lowtemperature in the condenser, since this is advantageous for theefficiency of the WHR system.

The medium may be further cooled in an air-cooled condenser at a frontportion of a vehicle. At this location the condenser has flowing throughit a cooling air flow at the temperature of the surroundings. Thecirculating medium may thus be cooled to a temperature close to thetemperature of the surroundings. The cooling which the circulatingmedium undergoes in such a condenser is closely related to thetemperature of the surrounding air but also to other parameters such asthe humidity of the air and the flow velocity of the cooling air throughthe charge air cooler. As the majority of these parameters depend on thestate of the surrounding air, it is difficult to regulate the cooling inan air-cooled condenser so that the medium is cooled to a desiredtemperature.

SUMMARY OF THE INVENTION

The object of the present invention is to propose a system capable ofconverting thermal energy to mechanical energy in an efficient way whileat the same time the condenser arrangement can be regulated and occupiesa relatively small amount of space in the vehicle.

This object is achieved with the arrangement of the kind mentioned inthe introduction which is characterised by the features indicated in thecharacterising part of claim 1. An advantage of an air-cooled condenseris that the medium can be cooled to a temperature close to thetemperature of the surroundings. Cooling the medium to as low atemperature as possible is advantageous for the efficiency of the WHRsystem. A disadvantage of an air-cooled condenser is that it isdifficult to regulate the cooling of the medium because the temperatureand moisture of the surrounding air may vary. Another disadvantage ofair-cooled condensers is that they require a relatively large fittingspace at a front surface of the vehicle for them to be able to providenecessary cooling of the medium by surrounding air. Cooling the mediumin a WHR system by means of a coolant-cooled condenser suffers thedisadvantage that the medium cannot be cooled to as low a temperature asin an air-cooled condenser. An advantage of a coolant-cooled condenseris that it is possible by relatively simple means to adjust thecoolant's temperature and flow through the condenser. The cooling of themedium can therefore be regulated. Another advantage of a coolant-cooledcondenser is that it can be made significantly smaller than anair-cooled condenser of similar capacity. A coolant-cooled condenser mayalso be situated substantially anywhere in a vehicle.

The present invention uses a condenser arrangement whereby the medium isfirst cooled in a coolant-cooled condenser and thereafter in anair-cooled condenser. A coolant-cooled condenser and an air-cooledcondenser situated in series in that order make it possible for thecondenser arrangement to have substantially all of the aforesaidpositive characteristics of the respective types of condenser. Where themedium undergoes a first step of cooling in a coolant-cooled condenser,the air-cooled condenser downstream may be made significantly smallerand occupy less space at the front portion of the vehicle. In theair-cooled condenser downstream the medium can be cooled to atemperature close to the temperature of the surroundings after havingalready been cooled to a low temperature in the coolant-cooledcondenser. The possibility of regulating the coolant temperature andflow through the coolant-cooled condenser means that the whole condenserarrangement can be regulated.

According to an embodiment of the invention, the vehicle is powered by acombustion engine cooled by coolant which circulates in a coolingsystem, and the coolant in said cooling circuit which cools the mediumin the first condenser will be at a lower temperature than the coolantwhich circulates in the engine's cooling system. It is possible, but notparticularly efficient, to use the relatively warm coolant in thecooling system which cools the engine to cool the medium in the firstevaporator. It is therefore appropriate to use a coolant which is at alower temperature. If the medium is cooled to a low temperature in thecoolant-cooled first condenser, the air-cooled second condenser need nothave a particularly high capacity and may therefore occupy little space.

According to a preferred embodiment of the invention, said coolingcircuit constitutes a separate cooling system from the cooling systemwhich cools the engine. Separate cooling system means that its coolantand components are separate from the engine's cooling system.Alternatively, said cooling circuit may form part of the cooling systemwhich cools the engine and in which the coolant is at a significantlylower temperature than in the main part of the engine. Said coolingcircuit comprises with advantage an air-cooled cooler in which thecirculating coolant is at least partly cooled by air at the temperatureof the surroundings. The coolant in the cooling circuit may therefore beat a very low temperature before it is used for cooling the medium inthe first condenser. Said cooling circuit comprises with advantage notonly the first condenser but also at least one cooler for cooling afurther medium. In vehicles there is a need to cool a plurality ofcomponents to a lower temperature than can be achieved with the coolantin the vehicle's combustion system. Such components may be therefrigerant in an AC installation, gearbox oil and electrical controlunits.

According to a preferred embodiment of the invention, the systemcomprises means for regulating the cooling of the medium in the firstcondenser. By regulating the coolant flow and temperature through thefirst condenser the cooling of the condenser arrangement can beregulated in a satisfactory way. The cooling in an air-cooled condenserdepends largely on the surrounding air's temperature, humidity and flowvelocity through the condenser. On the basis of information about theseparameters the cooling in an air-cooled condenser can be estimated butnot controlled. Both the flow and temperature of the coolant which coolsthe medium in a coolant-cooled condenser can be regulated. The flow maybe regulated by control of the coolant pump which circulates the coolantin the cooling circuit. The temperature of the coolant may be raised byactivating a heating unit or the like to supply thermal energy to thecooling circuit and may be lowered by disconnecting one or more of thefurther components which are cooled by the coolant which circulates inthe cooling circuit.

According to a preferred embodiment of the invention, the medium iscaused to absorb thermal energy in an evaporator from exhaust gaseswhich are led out from the engine in an exhaust line. The exhaust gasesfrom a combustion engine are a very good heat source which has a hightemperature. This thermal energy is normally lost to the surroundings.The medium may alternatively, or in combination, be caused to absorbthermal energy in an evaporator from exhaust gases which arerecirculated to the engine in a return line. The recirculating exhaustgases are normally cooled in at least one EGR cooler before they are ledto the engine. In this case the recirculating exhaust gases provideeffective cooling while at the same time part of the thermal energy inthem is converted to mechanical energy.

According to an embodiment of the present invention, the vehicle isprovided with a turbine adapted to being driven by the exhaust gases inthe exhaust line, and the system comprises at least one evaporator whichabsorbs thermal energy from exhaust gases in the exhaust line at alocation downstream of the turbine, and at least one evaporator whichabsorbs thermal energy from recirculating exhaust gases which are ledinto the return line at a location upstream of the turbine. In this casethe medium is warmed first by the exhaust gases in the exhaust lineupstream of the turbine, which are at the lower temperature, andthereafter by the recirculating exhaust gases in the return line, whichare at the higher temperature. The medium may in this case be warmed toa high temperature which results in good production of mechanical energyin the turbine. The system may comprise two evaporators arranged inseries in the return line. As the exhaust gases are at such a hightemperature in the return line, it may in many cases be appropriate tosubject the exhaust gases to two steps of cooling in order to utilise asmuch thermal energy as possible from the exhaust gases, while at thesame time the recirculating exhaust gases provide cooling to a lowtemperature. Having evaporators both in the exhaust line and in the linefor recirculation of exhaust gases make it possible for a relativelylarge part of the thermal energy in the engine's exhaust gases to beused for generating mechanical energy in the turbine.

BRIEF DESCRIPTION OF THE DRAWING

A preferred embodiment of the invention is described below by way ofexample with reference to the attached drawing, in which

FIG. 1 depicts an arrangement for conversion of thermal energy tomechanical energy in a vehicle.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION

FIG. 1 depicts a vehicle 1 powered by a supercharged combustion engine2. The vehicle 1 may be a heavy vehicle powered by a supercharged dieselengine. The exhaust gases from the cylinders of the engine 2 are led viaan exhaust manifold 3 to an exhaust line 4. The exhaust gases in theexhaust line 4, which will be at above atmospheric pressure, are led toa turbine 5 of a turbo unit. The turbine 5 is thus provided with drivingpower which is transferred, via a connection, to a compressor 6. Thecompressor 6 thereupon compresses air which is led into an inlet line 8via an air filter 7. A charge air cooler 9 is provided in the inlet line8. The charge air cooler 9 is situated in a region at a front portion ofthe vehicle 1. The purpose of the charge air cooler 9 is to cool thecompressed air which is led to the engine 2. The compressed air iscooled in the charge air cooler 9 by air which is forced through it by aradiator fan 10 and the draught caused by the vehicle's forwardmovement. The radiator fan 10 is driven by the engine 2 via a suitableconnection.

The engine 2 is cooled by coolant which circulates in a cooling system.The coolant is circulated in the cooling system by a coolant pump 11.The cooling system comprises a thermostat 12. The coolant in the coolingsystem is cooled in a radiator 13 fitted close to the radiator fan 10.The radiator 13 is situated downstream of the charge air cooler 9 withrespect to the direction of cooling air flow in the region. The coolingsystem comprises a line 14 which leads the coolant from the radiator 13to the engine 2. The coolant pump 11 is situated in a line 15. Thecooling system comprises a line 16 which leads the coolant from theengine 2 to the thermostat 12, and a line 17 which leads coolant fromthe thermostat 12 to the coolant-cooled first condenser 13. At timeswhen the coolant is at below a desired operating temperature, thethermostat 12 directs it from the line 16 to the engine 2 via the lines14, 15. At times when the coolant is at above a desired operatingtemperature, the thermostat 12 directs it from the line 16 to theradiator 13 via the line 17.

The vehicle 1 has a low-temperature cooling circuit with a circulatingcoolant which is at a considerably lower temperature than the coolant inthe engine's cooling system. The coolant is circulated in thelow-temperature cooling circuit by means of a coolant pump 18. Thecoolant is the low-temperature cooling circuit is intended to be cooledin a radiator 19 fitted in the region at the forward portion of thevehicle. The radiator 19 is situated upstream of the charge air cooler 9with respect to the direction of cooling air flow in the region. Thelow-temperature cooling circuit comprises a line 20 which leads coldcoolant from the radiator 19. The line 20 thereafter divides into fourparallel lines. The first parallel line comprises a cooler in the formof a condenser 21 to cool a refrigerant in an AC unit of the vehicle.The second parallel line comprises an oil cooler 22 to cool gearbox oilof the vehicle. The third parallel line comprises a cooler 23 to cool atleast one electrical control unit of the vehicle. The fourth parallelline comprises a condenser 24 in a WHR system. The coolant in thelow-temperature cooling circuit cools here a medium which circulates inthe WHR system. The medium is cooled in the condenser 24 to atemperature at which it condenses. The four parallel lines join togetherin a common line 25 which leads the coolant back to the radiator 19. Thelow-temperature cooling circuit comprises a control unit 26 which canregulate the operation of the coolant pump 18 and hence the flow ofcoolant through the low-temperature cooling circuit. The control unit 26may also, where necessary, activate a warming unit 27 to warm thecoolant if it is at too low a temperature. The warming unit 27 may be anelectrical unit or a heat exchanger in which the coolant in thelow-temperature cooling circuit is warmed by warm coolant from theengine's cooling system. If the temperature of the coolant in thelow-temperature cooling circuit needs to be lowered, the control unit 26can shut off the coolant flow through one or more of the mutuallyparallel coolers 21-24 by means of undepicted valves in the respectiveparallel lines. The control unit 26 can thus regulate the flow and thetemperature of the coolant which circulates in the low-temperaturecooling circuit. The control unit 26 may be a computer unit providedwith suitable software for this purpose.

The combustion engine 2 is provided with a system for recirculation ofexhaust gases known as EGR (exhaust gas recirculation). Suchrecirculation involves exhaust gases being mixed with the compressed airwhich is led to the engine's cylinders. The result is a loweredcombustion temperature and a lower content of nitrogen oxides NO_(x) inthe exhaust gases. Part of the exhaust gases from the exhaust line 4 isrecirculated through a return line 28. The return line 28 comprises anEGR valve 29 by which the exhaust flow in the return line 28 can beregulated to control the amount of exhaust gases recirculated. A controlunit 30 is adapted to controlling the EGR valve 29 on the basis ofinformation about the engine's current operating state. The control unit30 may be a computer unit provided with suitable software for thispurpose. The return line 28 comprises a first EGR cooler 31 in which therecirculating exhaust gases undergo a first step of cooling, and asecond EGR cooler 32 in which the recirculating exhaust gases undergo asecond step of cooling. After being cooled in said EGR coolers 31, 32,the recirculating exhaust gases are mixed with the compressed air in theinlet line 8, e.g. by means of a mixing device 33. The mixture ofcompressed air and recirculating exhaust gases is then led to therespective cylinders of the engine via a manifold 34.

The vehicle 1 is equipped with a WHR system for converting thermalenergy in exhaust gases led out from a combustion engine 2 to mechanicalenergy. The system comprises a line circuit 35 with a pump 36 which isadapted to pressurising and circulating the medium in the line circuit35. The medium is led initially by the pump 36 to a heat exchanger 37,which may be a recuperator, in which it undergoes a certain warming. Themedium is led from the heat exchanger 37 to an evaporator 32 which inthis case takes the form of a second EGR cooler in which the medium iswarmed by the recirculating exhaust gases. The medium mixture is led inparallel to an evaporator 38 which takes the form of a heat exchanger inwhich the medium is warmed by the exhaust gases in the exhaust line 4 ata location downstream of the turbine 5. The line circuit 35 thereafterjoins together so that the medium from the evaporators 32, 38 is ledjointly to a further evaporator 31 in the form of the first EGR cooler,in which it undergoes a further step of warming. When it leaves theevaporator 31, the medium is fully vaporised and extra-heated. Thegaseous medium leaving the first EGR cooler 31 is led to a turbine 39.The medium expands through the turbine 39, converting part of thethermal energy in the medium mixture to mechanical energy. The turbine39 drives in this case a generator 40 which converts the mechanicalenergy to electrical energy. The electrical energy is stored in anenergy store 41. The electrical energy stored in the energy store 41 maywith advantage be used to propel the vehicle or operate components ofit. Alternatively, the turbine 39 may be connected to a flywheel orsimilar mechanical energy storage unit which is connectable to thevehicle's power train. When the flywheel is connected to the powertrain, the vehicle is provided with extra propulsive power.

When the gaseous medium has expanded in the turbine 39, it will be at alower pressure and a lower temperature. The gaseous medium is led fromthe turbine 39 to the heat exchanger 37 in which it is cooled by theliquid medium from the pump 36. The gaseous medium is thereafter led tothe first condenser 24 in which it is cooled by the coolant in thelow-temperature cooling circuit. The medium is then led to a secondcondenser 42 situated downstream of the first condenser 24 with respectto the direction of its flow in the line circuit 35. The secondcondenser 42 is situated at a front portion of the vehicle 1 at alocation partly in front of the coolant-cooled second condenser 19. Themedium is cooled in the second condenser 42 by air at the temperature ofthe surroundings. The surrounding air is drawn through the secondcondenser 42 by the air fan 10 and the draught caused by the vehicle'sforward movement. The gaseous medium changes to liquid when it is cooledin the first condenser 24 and the second condenser 42. The medium maythus be cooled to a temperature close to the temperature of thesurroundings when it leaves the second condenser 42. The now liquidmedium is led from the second condenser 42 to the pump 36.

In this case, a condenser arrangement comprising a coolant-cooled firstcondenser 24 and an air-cooled second condenser 42 is used. The usuallyextra-heated gaseous medium is cooled in the coolant-cooled firstcondenser 24 to the condensation temperature of the medium. The mediumthus begins to condense in the coolant-cooled first condenser 24. Themedium is thereafter led to the air-cooled second condenser 42 in whicha remaining portion of the gaseous medium condenses. The liquid mediumin the air-cooled second condenser 42 is thereafter extra-cooled to atemperature close to the temperature of the surroundings. Since themedium has already undergone a good first step of cooling in thecoolant-cooled first condenser 24, it may be subjected to a second stepof cooling to a temperature close to the temperature of the surroundingsby means of a relatively small air-cooled second condenser 42 which maytherefore occupy relatively little space at the front portion of thevehicle.

In this case the flow and temperature of the coolant in thelow-temperature cooling circuit may thus be controlled by means of acontrol unit 26. The control unit 26 can therefore control the coolingof the medium in the coolant-cooled first condenser 24. On the basis ofknowing, for example, the temperature and humidity of the surroundingair and the velocity of the cooling air flow through the air-cooledcondenser 42, the control unit 26 can estimate the cooling of the mediumin the air-cooled condenser 42. This information enables the controlunit 26 to, for example, regulate the operation of the coolant pump 18in the low-temperature cooling circuit so that the medium fullyundergoes desired cooling in the first condenser 24 and the secondcondenser 42. The condenser arrangement thus makes it possible toregulate the cooling of the medium in the condenser arrangement, whichis not possible in an air-cooled condenser.

Where the air-cooled condenser 42 occupies relatively little space atthe front portion of the vehicle, the coolant in the low-temperaturecooling circuit can also be partly cooled by air at the temperature ofthe surroundings. This makes it possible for the low-temperature coolingcircuit to be at a low temperature when it leaves the radiator 19. Thecooling of the medium in the first condenser 24 and in the mutuallyparallel coolers 21, 23 thus becomes effective.

In this case, thermal energy is absorbed from the engine's exhaustgases. The exhaust gas from a combustion engine is a very good heatsource from which to recover thermal energy. If the engine 2 is a dieselengine, the exhaust gases may be at a temperature of about 600-700° C.upstream of the turbo unit's turbine 5 at times when the engine 2 isunder heavy load. The recirculating exhaust gases in the return line 28may thus initially be at this temperature. The temperature of theexhaust gases downstream of the turbine 5 may be of the order of200-300° C. The medium is in this case warmed initially in two mutuallyparallel evaporators 32, 38. In the evaporator 38, the medium is warmedby exhaust gases which have expanded through the turbine. In the otherevaporator, the medium is warmed by exhaust gases in the return line 28after having undergone a first step of cooling. In both cases the mediummay be warmed by exhaust gases which are at a similar temperature, whichmay thus be 200-300° C. All of the medium is then led to evaporator 31,in which it is warmed by recirculating exhaust gases in the return line28. The recirculating exhaust gases may thus here be at a temperature ofabout 600-700° C. However, the media used in WHR systems have an uppertemperature which should not be exceeded. Suitable dimensioning of theevaporators 31, 32, 38 will make it possible for medium to be warmed toa high temperature appropriate to the specific medium before it is ledto the turbine 39. A high temperature of the medium led to the turbinemakes it possible to produce a large amount of mechanical energy.

The invention is in no way restricted to the embodiment to which thedrawing refers but may be varied freely within the scopes of the claims.

1. A system for converting thermal energy to mechanical energy in avehicle comprising: a cooling circuit configured for circulating acirculating coolant; a line circuit, a pump for recirculating a mediumin the line circuit, at least one evaporator at the line circuit inwhich the medium is caused to absorb thermal energy from a heat sourcefor vaporising the medium; a turbine configured and operable for havingthe vaporised medium pass by the turbine in order to cause the turbineto be driven by the vaporised medium in order for the turbine togenerate mechanical energy; a condenser arrangement configured andoperable for then having the medium pass by the condenser arrangementand in which the medium is caused to give off thermal energy so that themedium condenses, the condenser arrangement comprising: a firstcondenser in which the medium gives off thermal energy to the coolantwhich circulates in the cooling circuit; and a second condenserdownstream of the first condenser with respect to a direction of flow ofthe medium in the line circuit and in which the medium gives off thermalenergy to air at the temperature of the surroundings.
 2. A systemaccording to claim 1, furthering comprising a combustion engine poweringthe vehicle; a cooling system which circulates a second circulatingcoolant for cooling the engine; and the cooling circuit being configuredsuch that the coolant in the cooling circuit cools the medium in thefirst condenser such the medium in the first condenser is at a lowertemperature than the second coolant which circulates in the enginecooling system.
 3. A system according to claim 2, wherein the coolingcircuit comprises a second cooling system that is separate from thecooling system which cools the engine.
 4. A system according to claim 1,wherein the cooling circuit further comprises an air-cooled cooler inwhich the circulating coolant is at least partly cooled by air at thetemperature of the surroundings.
 5. A system according to claim 4,wherein the cooling circuit comprises the first condenser and at leastone further cooler for cooling a further medium.
 6. A system accordingto claim 1, further comprising a device configured for regulating thecooling of the medium in the first condenser.
 7. A system according toclaim 2, further comprising an exhaust line for exhaust from the engine,at least one of the evaporators being at the exhaust line, so that themedium is caused to absorb thermal energy in the at least one evaporatorfrom exhaust gases which are led out from the engine in the exhaustline.
 8. A system according to claim 7, further comprising a return linelocated and configured for returning exhaust gases from the exhaust lineto the engine, at least one of the evaporators being at the return line,so that the medium is caused to absorb thermal energy in at least one ofthe evaporators from exhaust gases in the return line.
 9. A systemaccording to claim 8, further comprising the vehicle includes a secondturbine located and configured to being driven by the exhaust gases inthe exhaust line; the system comprises at least one additionalevaporator configured for absorbing thermal energy from exhaust gases inthe exhaust line and at a location downstream in the exhaust gases flowof the second turbine, and the at least one additional evaporatorabsorbs thermal energy from recirculating exhaust gases which are ledinto the return line at a location upstream in the exhaust gases flow ofthe second turbine.
 10. A system according to claim 9, furthercomprising at least two of the evaporators arranged in series in thereturn line.
 11. A system according to claim 2, further comprising areturn line located and configured for returning exhaust gases from theexhaust line to the engine, at least one of the evaporators being at thereturn line, so that the medium is caused to absorb thermal energy inthe at least one evaporator from exhaust gases in the return line.